Several techniques and systems have been developed for correcting and stabilizing the spine and for facilitating fusion at various levels of the spine. In one type of system, a bendable rod is disposed longitudinally along the length of the spine or vertebral column. The rod is preferably bent to correspond to the normal curvature of the spine in the particular region being instrumented. For example, the rod can be bent to form a normal kyphotic curvature for the thoracic region of the spine, or a lordotic curvature for the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along the length of the spinal column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of a vertebra. For instance, one such fixation element is a hook that is configured to engage the laminae of the vertebra. Other prevalent fixation elements include spinal screws or bolts that can be threaded into vertebral bone.
In one typical procedure utilizing a bendable spinal rod, the rod is situated on opposite sides of the spine or spinous processes. A plurality of fixation elements is attached to a portion of several vertebral bodies. The rods are then affixed to the plurality of fixation elements to thereby apply corrective and stabilizing forces to the spine.
One example of a rod-type spinal fixation system is the Cotrel-Dubosset/CD.RTM. Spinal System ("the CD.RTM. System") sold by Sofamor Danek Group, Inc. The CD.RTM. System provides a variety of fixation elements for engagement between an elongate rod and the spine. In one aspect of the CD.RTM. System, the fixation elements include a body that defines a slot within which the elongate rod is received. The slot includes a threaded bore into which a threaded plug is engaged in order to clamp the rod within the body of the fixation element. The CD.RTM. System includes hooks and bone screws with this "open-back" configuration. Details of this technology can be found in U.S. Pat. No. 5,005,562 to Dr. Cotrel. One benefit of this feature of the CD.RTM. System is that the fixation element is positioned directly beneath the elongate rod. This helps reduce the overall bulkiness of the implant construct and minimizes surgical trauma to surrounding tissue. However, the fixation elements of the CD.RTM. System are capable only of pivoting about the longitudinal axis of the elongate rod to achieve variable angular positions relative thereto. While this type of system is acceptable for many spinal pathologies, other cases require the fixation elements be angularly oriented in multiple planes relative to the axis of the rod. In other words, the fixation element must sometimes be allowed to pivot relative to the rod in a generally cone-shaped path. Screws of this type have been referred to as poly-axial or multi-axial bone screws.
Various poly-axial bone screw designs have been disclosed, generally including a receiver member configured to connect a bone screw having a curvate head to an elongate rod. The receiver member typically has a threaded portion adapted to threadedly engage a nut or set screw to thereby grip the head of the bone screw and connect the screw to the rod at a desired angular orientation. However, nuts and set screws have been known to have a tendency to back-out in in-vivo situations. This could likely cause the poly-axial bone screw assembly to loosen, thus requiring additional surgery. Moreover, the nuts and set screws may strip or gall, and their installation can be quite cumbersome due to the limited amount of space available to manipulate the tools necessary to drive the nuts and set screws into their engaged position. Furthermore, poly-axial bone screw designs of the past have typically required a multiplicity of parts that often make complete fixation of the bone screw a fairly complex process.
In recent years, a special material known as "shape-memory alloy" has been used in the construction of various mechanical devices. This type of material is an alloy of known metals, such as copper and zinc, nickel and titanium, silver and cadmium, and others, that are known to exhibit a "shape-memory" characteristic in which a particular component formed of a shape-memory alloy ("SMA") is capable of reforming to a "memorized" shape at certain temperatures. This phenomena occurs when the SMA alloy changes from a martensitic crystal phase to an austenitic crystal phase. In the martensite stage, the SMA is relatively weak and pliable. As the temperature of the SMA component is increased above its transformation temperature range, the SMA transforms to an austenitic stage and the material becomes relatively strong with super-elastic properties. Generally, the strength and super-elastic characteristics of a shape-memory material tend to increase toward the high temperature end of the transformation temperature range, and decrease toward the low temperature end. While there are many alloys that exhibit shape-memory characteristics, one of the more common SMAs is an alloy of nickel and titanium. One such well known alloy is Nitinol.RTM., which has proven to be highly effective for devices to be placed within the human body because its transformation temperature range falls between room temperature and normal human body temperature.
There is a general need in the industry to provide an advanced coupling device which uses shape-memory technology to connect two or more members. There is also a more specific need to provide an improved multi-axial bone engaging fastener using shape-memory technology. This need also encompasses a goal of minimizing the profile and bulk of the components used to attach the bone engaging fastener to a vertebra and to further connect the bone engaging fastener to an elongate member. Furthermore, it is desirable to reduce the number of components that must be manipulated by the surgeon during a surgical procedure. The present invention meets these general and specific needs and provides other benefits and advantages in a novel and unobvious manner.